Rebels fighting government forces in Syria's commercial capital Aleppo "will definitely be defeated", Syrian Foreign Minister Walid al-Muallem said on an unannounced visit to key ally Iran.

"We believe that all the anti-Syrian forces have gathered in Aleppo to fight the government... and they will definitely be defeated," he told a joint news conference on Sunday in Tehran with Ali Akbar Salehi, the Iranian foreign minister.

His assertion came as Syrian troops, backed by air power and tanks, pushed on in the second day of an assault on the northern city of Aleppo, sparking international fears of a humanitarian catastrophe.

Salehi warned that, if the Syria conflict worsened and the regime of President Bashar al-Assad fell, the consequences "would engulf the region and eventually the whole world".

He added that "it's naive and illusory to think that, if a [power] vacuum opens up in Syria and the government changes, a new government could be easily established".

He urged Syria's neighbours to think hard about the situation, otherwise "everyone will lose".

'Global plot'

Syria and Iran accuse several countries in the region, notably Saudi Arabia, Qatar and Turkey, of aiding the rebels in their fight against Assad's forces, with direction from the United States and in support of Israeli goals.

"Syria is the target of a global plot whose agents are the countries in the region," Muallem said.

"The reality is that, for a month now, there has been a propaganda war waged by the United States, Europe and certain Arab countries," he said.

The Syrian foreign minister, whose visit to Tehran on Sunday was unscheduled, was to speak with several Iranian officials about the situation in his country, according to the Iranian foreign ministry.

After Salehi, he was to see Saeed Jalili, the head of Iran's supreme national defence council and a close aide to supreme leader Ayatollah Ali Khamenei, as well as parliament speaker Ali Larijani.

Iran is the staunchest ally of the Syrian regime. It has provided humanitarian aid and diplomatic backing but denies reports that it has also given military support.